Full-Scale Seismic Testing of Piles in Improved and Unimproved Soft Clay

A full-scale field investigation was performed to determine the effects of soil improvement on the seismic resistance of piles in soft clay. A soil improvement method, called cement deep soil mixing (CDSM), was used to improve soil supporting a standard 324 mm diameter steel pipe pile subjected to simulated earthquake lateral loads. An identical pile in unimproved clay was also tested to determine the effects of the soil improvement. Compared to the unimproved pile, the CDSM technique showed a 42% increase in pile lateral strength, a 600% increase in effective elastic stiffness, and a 650% increase in average equivalent damping ratio. The pile in improved soil reached its lateral capacity at a head displacement of 0.1 m, at which point the critical region at the base of the pile above the improved ground experienced buckling and subsequent fracture due to low cycle fatigue.